1) Field of the Invention
The present invention relates to a toner image transferring device for electrostatically transferring a toner image from a toner image carrying body such as a photosensitive drum, a dielectric drum, a dielectric belt or the like to a sheet or paper. The present invention also relates to an electrophotographical recording apparatus such as a laser printer in which such a toner image transferring device is incorporated.
2) Description of the Related Art
For example, the toner image transferring device is incorporated in an electrophotographic printer which typically carries out the processes of: producing a uniform distribution of electrical charges on a surface of a rotary photosensitive drum; forming an electrostatic latent image on the electrically charged surface of the photosensitive drum by optically writing an image thereon with a laser beam scanner, an LED (light emitting diode) array, an LCS (liquid crystal shutter) array or the like; visually developing the electrostatic latent image with a developer, i.e., toner, which is electrically charged to be electrostatically adhered to the electrostatic latent image zone; electrostatically transferring the developed visible image or toner image from the drum to a sheet or paper; and fixing the transferred image on the sheet or paper.
In the transferring process, the transfer of the toner image from the photosensitive drum to the paper is carried out by feeding an electric charge to the paper. Conventionally, the toner image transferring device comprises an electric discharger such as a corona discharger for feeding the electric charge to the paper. Namely, the discharger is disposed in the vicinity of the photosensitive drum, the paper is introduced into a clearance therebetween, and the discharger gives the paper an electric charge having a polarity opposite to that of the electric charge of the toner image, whereby the toner image is electrostatically transferred from the drum to the sheet or paper.
The electric discharger has an inherent defect that ozone is produced during the energizing thereof. Not only is ozone injurious to the health, but also it causes a premature deterioration of the photosensitive drum and other parts of the printer. Also, the use of the electric discharger results in an increase in the production cost of the printer, because it must be provided with a high voltage electric power source for the electric discharger and an ozone filter for preventing an ozone leakage.
For this reason, a conductive roller type toner image transferring device has been recently used in the electrophotographic printer. This type of toner image transferring device comprises a conductive elastic transfer roller pressed against a surface of the photosensitive drum and rotated at the same peripheral speed as the drum, and while a sheet or paper is passed through a nip between the conductive transfer roller and the drum, the conductive transfer roller is supplied with an electric energy to give the paper an electric charge having a polarity opposite to that of the charge of the toner image. Note, in general, an electric source for supplying the electric energy to the conductive transfer roller has a lower power than that of the electric discharger.
Note, the quality of a transferred toner image depends upon an efficiency of the transfer of the toner image from the photosensitive drum to the paper, and the toner transfer efficiency is governed by various factors such as a nature of the electric source, a resistivity of the conductive transfer roller, and a nature of a paper to which the toner image is transferred. Accordingly, these factors should be determined so that it is possible to obtain a reasonable transfer efficiency to produce a transferred toner image quality at least passable in practical use. If only one kind of paper having the same size is used in the printer, it is very easy to determine other factors such that the reasonable transfer efficiency can be obtained, but when various kinds of papers having a different size are actually used in the printer, it is very difficult to suitably and properly determine the nature of the electric source and the resistivity of the conductive transfer roller such that the reasonable transfer efficiency can be obtained with respect to the papers having a different size.
Also, the conductive transfer roller has an inherent defect in that it is easily polluted by residual toner particles on the photosensitive drum. Furthermore, when a paper cannot reach the nip between the conductive transfer roller and the photosensitive drum due, for example, to jamming, the toner image is directly transfered to the conductive transfer roller, and thus this roller is greatly polluted by the transferred toner image. Of course, the polluted conductive transfer roller leaves a stain on a rear surface of a paper during a passage of the paper through the nip between the roller and the drum, and accordingly, the conductive transfer roller must be frequently cleaned in such a manner that it leaves no stain on the rear surface of the paper. Nevertheless, a conventional conductive transfer roller type toner image transferring device fails to clean the polluted conductive transfer roller in the manner such that it cannot leave any stain on the rear surface of the paper, as discussed in detail hereinafter.